Obtaining information about vehicle flow within a navigable network, e.g. a road network, is important in a navigation system. Flow data is indicative of the speeds of travel of vehicles within the network. Flow data may be used in a variety of ways. For example, flow data may be used to determine traffic data for the network. A comparison of actual vehicle speeds to the expected speed of travel or free flow speed of travel at an applicable time may enable regions of reduced flow, e.g. traffic jams, to be identified. Flow data may also be used in routing. Flow data may be used to determine traversal times for elements of the navigable network, enabling routes, e.g. fastest routes, to be generated based on current traversal times. Flow data for a navigable network may provide a real-time picture of vehicle speeds within the entire network. The flow data may be in terms of absolute speeds, i.e. average speeds, of travel, or relative speeds of travel, e.g. relative to an applicable free flow speed. This is in contrast to traffic data, which typically relates to individual events within the network which affect traffic flow. For example, traffic data may be indicative of traffic jams having a start point, an end point, and a particular speed reduction associated therewith. In addition to current flow speed data, predicted flow speed data for one or more future times may alternatively or additionally be generated and used in a similar manner.
While flow speed data may be of great utility, providing such data in a form appropriate for transmission to devices, e.g. to servers or navigation devices, requiring a flow speed data feed presents some challenges. Flow speed data is typically provided in respect of links. As described below, a link is predefined by reference to an electronic map comprising a plurality of navigable segments representing navigable elements of the navigable network. A link may be defined by at least a portion of one or more navigable segments of the map extending between certain fixed reference points of the navigable network. However, flow speeds, i.e. average vehicle speeds, within the network may exhibit relatively small scale variations. For example, average vehicle speeds may vary along the length of navigable links of the network. In order to try to capture such variation, one possibility would be to divide each navigable link of the network into sections of unit length, and to determine flow speed data in respect of each section. The unit length may be set to be sufficiently small to try to capture variation in flow speed along the length of a link. However, such methods may be highly data intensive, requiring large amounts of flow speed data to be generated. This places significant demands on the processing and storage capabilities of devices receiving the data, and requires a large amount of bandwidth for transmission. The Applicant has realised that there remains a need for improved methods and systems for providing flow data in respect of a navigable network.